The goal of the proposed research is to increase our understanding of the mechanisms by which vertebrate nervous systems process sensory information. This research program focuses on the roles of modulatory inputs to sensory processing networks; the function of feedback connections within a network and the effects of inputs from one sensory system in determining the processing characteristics of other systems will be studied. Sensory systems are typically described as consisting of a series of hierarchically arranged nuclei or stations which sequentially process the information as it ascends from the periphery through higher- order nuclei. However, in addition to this ascending information stream, higher centers also communicate with lower, and the volume of information carried via these descending pathways is typically massive. Roles proposed for these feedback pathways include modulation of the gain of ascending signals, gating functions, alterations in the receptive field properties, and changes in spatial and temporal filter characteristics of target neurons. The descending pathways are also thought to be involved in improving signal-to-noise ratios and enabling an organism to attend to specific stimuli. Losses in the normal ability to filter incoming sensory information is demonstrated to be associated with diseases such as schizophrenia and deficits in the ability to attend to stimuli may underlie a variety of learning disabilities. Possible roles for descending information flow in sensory systems abound but demonstrated examples of these roles are few, and instances in which the cellular mechanisms involved are understood are fewer still. Modulatory inputs arising in other sensory systems are also likely to be important sources of information required for correct interpretation of a given stimulus pattern. Proprioceptive inputs conveying information about the spatial relations among regions of the body can, for example, provide information needed for correct interpretation of a variety of other sensory inputs. Basic mechanisms of the descending control of sensory processing will be studied using a lower-vertebrate "model" sensory system, the electrosensory system of weakly electric fish. The first-order processing nucleus, the electrosensory lateral line lobe (ELL), receives massive descending projections from higher electrosensory nuclei plus proprioceptive inputs in addition to the receptor afferent projection. The roles of these modulatory inputs in shaping the sensory processing characteristics of ELL output neurons will be studied using single cell neurophysiological techniques plus pharmacological manipulations of the system. The ELL was chosen for study because its anatomy as well as that of the regions supplying modulatory inputs is well described, and because a large amount of physiological and behavioral data pertinent to the mechanisms to be studied is available. The proposed studies will increase our knowledge of the roles of these modulatory inputs in this particular system and will contribute to a greater understanding of the cellular mechanisms by which the modulatory effects are generally achieved.